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Roger Arnold is a former software engineer and systems architect. He studied physics, math, and chemistry at Michigan State University's Honors College , where he graduated in 1967. A US Army veteran (courtesy of the...

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Virgin Earth Challenge: How Goes It?

Background

In February of 2007, Richard Branson and Al Gore announced the Virgin Earth Challenge — a prize of $25 million “for whoever can demonstrate a commercially viable design which results in the permanent removal of greenhouse gases out of the Earth’s atmosphere”. In 2011, a list of 11 finalists was released. None were judged to have demonstrated the level of commercial viability that awarding of the prize required, so the competition period was extended.

One point about the VEC needs emphasis. The rules of the challenge make it clear that, while reducing GHG emissions is a vitally important goal that needs to be pursued, that’s not what the VEC is about. It’s about practical methods for removal and sequestration of existing greenhouses gases from the atmosphere. Advances in wind and solar power, as well as energy efficiency and conservation, are laudable, but they’re not candidates for the VEC. Nor are point source carbon capture technologies, per se. Developments in those areas will help to reduce GHG emissions, but they can’t help with removal of greenhouse gases from the atmosphere once the gasses are there. They can’t (directly) address distributed carbon emissions or help with the future GHG overshoot problem we’re facing. In the jargon of climate change, the VEC is about CDR (CO₂ removal), not point source CCS (CO₂ capture and sequestration).

Fair enough. But it’s worth noting that there are approaches that blur the line between CDR and point source CCS. One of those is well recognized. That’s Bio-Energy with Carbon Capture and Storage (BECSS). It’s one of the CDR processes recognized by the VEC and represented in the list of finalists. But there are other CDR technologies that depend on CCS and are not represented among the VEC finalists. A prominent example would be carbon-neutral production of Portland cement. The carbon emissions of cement production are all up front; once it’s been produced, ordinary concrete is a carbon-negative product. Over its lifetime, it will absorb atmospheric CO₂ in amounts nearly equal to what was emitted by calcining the limestone used in its production. Unfortunately, point-source capture technologies that would coincidentally make carbon-neutral production of cement economically feasible are excluded from consideration for the VEC.

Delays and Difficulties

In the time since the list of finalists was announced, Kilimanjaro Energy, one of the original finalists, has folded. However it’s technology for direct air capture of CO₂ has been acquired by a new company that continues efforts to commercialize it. That company, Carbon Sink, has subsequently replaced Kilimanjaro Energy in the list of finalists. The other listed candidates continue working toward commercial viability and remain in the running.

The long delay in selecting a winner reflects the difficulty of the challenge. The winner must convince the judges that its approach is scalable to at least a billion tonnes of CO₂ per year and commercially viable at that scale. Many candidate technologies can make a case for commercial viability as niche applications. However, the markets on which their viability depends — e.g., selling CO₂ to greenhouse operators — aren’t currently that large. So the work goes on.

Despite the absence of a winner, the list of finalists is revealing. The technologies they are pursuing provide a window on the current state of CDR. That window may give us some insight as to what the future of carbon mitigation may hold — at least in the near term.

Approaches

The eleven VEC finalists can be classified by the general approaches they are pursuing. The table below lists the different approaches and the associated companies.

Biochar

Biochar is a relatively low-tech approach inspired by the terra preta soils found in the Amazon basin. These black, fertile soils were created in pre-Columbian times by indigenous farming cultures. They mixed wood char, crushed bone, and manure into the otherwise relatively infertile Amazonian soil to build crop beds. The wood char, though not a fertilizer per se, served to buffer nutrients from the bone meal and manure. It apparently served as a soil analog of a coral reef. Its porous structure and nutrient buffering surface area created a favorable microenvironment for communities of soil fungi and other organisms that aided soil fertility.

Terra preta soils, once well established, appear to be self-sustaining. So long as crop cover protects them from wind and water erosion, they maintain their high level of soil carbon and productivity long after additions of the materials that built them have stopped. In fact they gradually increase in depth as new material composts. In the Amazon basin, thick terra preta soil beds built as far back as 450 BCE remain productive and highly valued by local farmers to this day.

Terra preta soils were initially thought to be peculiar to the warm, wet environment of the Amazon basin. Research has shown, however, that similar results can be obtained in temperate regions by amending soils with formulations of biochar and other ingredients tailored to local soil and crop conditions. The amount of carbon that can potentially be stored in this manner is huge; the amount currently stored as soil carbon has been estimated as 2,300 GT, nearly three times the 800 GT of carbon now present in the atmosphere. If soil carbon could be increased globally by an average of just 10%, it would sequester enough carbon to return atmospheric CO₂ to pre-industrial levels.

The issue with biochar then is not the amount of carbon it could ultimately sequester in the soil; it’s (surprise!) economics. There’s little doubt that a well designed program of soil building, incorporating use of biochar as an element, would be an effective way to sequester carbon while providing long term economic value to farmers. It would boost crop yields while reducing the amount of fertilizer needed. It would also reduce water runoff and nutrient leaching while improving drought resistance. On the other hand, biochar is costly to produce and distribute in the amounts needed, and it may take decades for the considerable investment in soil quality to pay off financially.

The key to success for biochar will come down to technology for producing it from local resources, and dissemination of knowledge for how to employ in in a broader program of soil building. A sense of the complexities can be found in a document from the International Biochar Initiative: Guidelines on Practical Aspects of Biochar Application to Field Soil in Various Soil Management Systems. The three VEC finalists developing biochar display the diversity of product and business strategies possible for addressing these complexities.

BECCS

BECCS is a CDR approach that was called out in the IPCC’s 5th Assessment Report (AR5) as likely to be needed in the period after 2050 if the world is to have any chance of remaining within the mandated 2 °C ceiling for global warming. The assumption is that CCS will already be widely implemented for reducing CO₂ emissions from coal-fired power plants. Growing biomass is an easy way to pull CO₂ out of the atmosphere; burning some of it along with or instead of coal then provides a low cost way to sequester its carbon content.

That’s one path, at least. There’s an alternative that bypasses the need for the expensive large scale CCS system that would be needed for a coal-fired power plant. That’s to tap the CO₂ gas stream from an ethanol fermentation plant. It’s nearly pure CO₂ already; all it needs is compression and transport to a well for injection into a deep saline aquifer or depleted oil field. The volume of flow is much smaller than it would be for a large power plant, allowing relatively low cost demonstration projects to be built. That’s the approach being pursued by Sweden’s Biorecro. They are participating in three government-funded research projects in the US. It’s doubtful, however, that CO₂ capture from fermentation could scale to a billion tonnes of CO₂ per year.

Direct Air Capture

Direct air capture removes CO₂ from the open atmosphere, rather than a power plant’s flue stack. The advantage is that a DAC plant can be located anywhere that’s convenient; it doesn’t matter how and where the CO₂ the plant is capturing originated. For geological sequestration or for EOR, the plant can be co-located near the injection wells. No long distance CO₂ pipelines are needed.

The disadvantage is that the CO₂ being captured is 400 times less concentrated than in a power plant’s flue stack. That raises the thermodynamic minimum energy needed to capture and compress it to a pure CO₂ stream. But it also increases by a factor of 400 the gross amount of air that needs to contact the surface of the sorbent to capture the same amount of CO₂.

How much of a problem that may be depends on the technology employed. In approaches I previously wrote about (here, here, here, and here) it’s a non-issue. Those approaches involved boosting alkalinity in ocean surface waters. Because the oceans have such enormous surface areas in contact with the atmosphere, neither the concentration of CO₂ nor the concentration of added alkalinity matter much. Only the total amount of alkalinity added matters — at least to a first approximation. But for an individual DAC plant intended to support, say, a major enhanced oil recovery operation, the sheer volume of air that must contact the sorbent material is daunting.

As an example, the array shown in this rendering from Carbon Engineering houses the air contactors for a plant they are hoping to build in the near future. The array is the height of a 5-story building, and about a tenth of a mile long. This array could potentially capture maybe 300 tons of CO₂ per day, or 100,000 tons per year in round numbers. To hit the billion tons per year that the Virgin Earth Challenge is aiming for would require around 10,000 such plants operating full time. It’s not impossible, but Carbon Engineering’s own estimates project a cost of around $100 per ton of captured CO₂. There’s currently no significant market for CO₂ at that price.

Note that, large as the above air contactor array may seem, Carbon Engineering touts the relatively small size and high efficiency of their air contactor (compared to possible alternatives) as one of their chief competitive advantages. The contactor is only a part of the overall capture system, however. Equally important is the chemical processing that releases a pure CO₂ stream from the exposed sorbent and regenerates the sorbent for another cycle. Carbon Engineering’s process involves precipitation of calcium carbonate, which is rinsed and then heated to 800 to 900 C for calcining. Calcining releases CO₂ gas, and leaves solid calcium oxide (CaO). That step is part of what I termed, in a previous article, the “brute force” approach to ocean capture of CO₂. The difference is that where I called for the CaO to be slaked and broadcast over ocean surface waters to raise their pH, Carbon Engineering uses it to regenerate the potassium hydroxide solution that they use in their contactor.

Three of the other companies competing in the DAC field use proprietary sorbent processes about which I’ve been unable to learn much. They all seem to involve sorbents that use organic molecules with amine groups to capture and hold CO₂ molecules. That’s also how the most established approach to CO₂ capture works. The sorbent in that case is liquid monoethanolamine (MEA). It’s long been used in the oil and gas industry for scrubbing CO₂ from raw natural gas. However, it requires inconveniently high temperature to strip the adsorbed CO₂ from the MEA. For “polishing” natural gas, that’s not a significant issue. For the amounts of CO₂ contained in flue gases, it is. Providing the high temperature steam to do the stripping represents a major parasitic drain on the plant. The three VEC finalists that appear to be using proprietary amine-based sorbents for DAC have alternatives to liquid MEA. They’re able to absorb CO₂ at ambient temperatures and strip it using only low grade heat (< 80° C).

The DAC approach that I find most intriguing is the low energy humidity swing method used by Carbon Sink through its Infinitree subsidiary. It’s an adaptation of the process discovered by Prof. Klaus Lackner and colleagues while Lackner was at Columbia University’s Earth Institute. What’s interesting about it is that it requires almost no energy input. It uses the implicit energy that exists in the normal disequilibrium between water and air at less than 100% humidity. That disequilibrium is a renewable energy source that’s rarely even recognized, much less tapped.

The sorbent in the moisture swing process adsorbs CO₂ from the air when the sorbent is dry; it exchanges the adsorbed CO₂ in favor of water vapor when it is wetted or exposed to humid air. After the CO₂ has been released, the sorbent material is allowed to dry in ambient air, after which it is able to take up more CO₂. The process is well suited to enhancement of CO₂ levels in warm humid greenhouses.

Enhanced weathering

Chemical weathering of silicate minerals in rocks by rain water is “nature’s way” of achieving CDR. Traces of atmospheric CO₂ dissolve in raindrops, turning them mildly acidic. The acidic drops dissolve tiny amounts of minerals on exposed rock surfaces that they fall on or trickle over. The dissolved minerals are carried by runoff as bicarbonate salts and silicic acid.

Natural chemical weathering is a slow process. The amount of CO₂ that dissolves in raindrops is minute, and the acidity of the drops is weak. After a rock surface has been exposed and subjected to any degree of weathering, it acquires a varnish of less soluble material that protects it from further weathering. The surface must be abraded by blown dust or particles in running water to enable chemical weathering to continue.

Some minerals are softer and more prone to weathering than others. Among the more easily and rapidly weathered minerals is olivine. It’s a common mineral that can be mined and crushed to make green olivine sands of the sort pictured here. There are places where the physical weathering of rocks with high olivine content create natural olivine sands. They are transient features in nature, however, because chemical weathering dissolves them over the geologically brief time scale of centuries to millennia. Accordingly, one of the simpler and more direct approaches to CDR is to mine and crush olivine source rocks, producing olivine sands. The artificially created sands can then be spread in surf zones along the coasts, where agitation by waves will abrade the particles and prevent buildup of the rock varnish that impedes chemical weathering. This approach has been demonstrated on a small scale by the Olivine Foundation, It works, and has earned the foundation a place in the VEC finalist list. The only problem, once again, is economics. In the absence of a price on carbon emissions (and corresponding credit for CDR), there’s no economic return to cover the costs of mining, crushing, and distributing the olivine sands.

Grassland Restoration

To me, the most surprising candidate in the list of VEC finalists is the Savory Institute. It advocates for restoration of high quality native grasslands through better management of grazing. It specifically does not call for reduced grazing and shifting dietary preferences in developed nations away from meat. Rather, it calls for more informed management of the way lands are grazed. The objective is to simulate the manner in which wild grasslands were grazed by great herds of bison, antelope, and other grass eaters in days before the great wild herds were decimated.

The grazing pattern that the Savory Institute promotes is one of heavy grazing in a selected area for a short time, followed by much longer period for recovery and regrowth. The pattern favors deep-rooted perennial grasses at the expense of annuals and woody plants that are damaged by trampling. In the wild, it’s the natural result of herd migration. In domestic settings, it must be imposed by fences and rotation of livestock among fenced sub-pastures.

The heavy grazing episodes destroy annual weeds and grasses, bushes, and sapling trees, without really damaging deep-rooted perennial grasses. After the grazing herds move on, the perennial grasses draw on energy stored in their roots and the bounty of manure left by the grazers to quickly regrow surface foliage. The symbiotic relationship between grazing herds and perennial grasses is something that evolved over millions of years, so it shouldn’t be surprising that the pattern works. What’s surprising (to me) is that there would be enough soil carbon stored in mature native grasslands to make a difference in atmospheric CO₂ levels. If that’s the case, then modern farming has depleted soil carbon reserves and been a larger contributor to rising CO₂ in the atmosphere than is usually acknowledged.

Bottom Line

The bottom line for CDR technologies that the Virgin Earth Challenge was intended to stimulate is a typical good news / bad news type of thing:

The good news is that there is a broad diversity of technically feasible approaches. Some of the approaches are economically viable, even without the aide of a meaningful price on carbon emissions. And some are scalable to levels that would make a real difference for mitigation of global warming;

The bad news is that the two groups are disjoint. So far, at least, those that are economical are not sufficiently scalable, and those that are scalable are not sufficiently economical.

Work continues on both technical and business models to find scalable CDR solutions that are economically viable in the current policy regime. But if we really hope to avoid the fate of the proverbial frog in the slowly heating pot of water, I think the odds are long against finding a magic bullet. The only bullet we can count on is the one that economists have long been telling us we’ll have to bite if we’re serious about reducing CO₂ levels in the atmosphere: a meaningful price on carbon emissions.

In what I heard about bio-char from Danny Day — one of its earliest and most ardent proponents — the pyrolysis temperature is critical. It needs to be high enough to achieve relatively complete pyrolysis, but low enough to leave a highly porous carbon skeleton reflecting the cellular structure of the source biomass without significant “graphitization”. The biochar seems to achieve its beneficial effects, in part, by serving as a sort of “coral reef” for mycorrhizea and other soil microfauna. Those organisms speed decomposition of dead organic matter in the soil and facilitate uptake of nutrients by plant roots.

I don’t know if super-torrefaction in a bath of molten salt would produce a bio-char with favorable properties. Since the melting point of salt baths can be adjusted over a wide range, I don’t see any reason it couldn’t. But I’m no expert on that particular subject.

In any case, what you say about “economically challenging” is probably true. The improvement in soil fertility provides a long-term benefit that would eventually pay off, even without credit for carbon sequestration. But it takes massive application; without credit for carbon sequestration, the payback time could be impractically long for the fast-changing culture in which we’re immersed.

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